The present invention relates to a power supply circuit, a display driver, an electro-optical device, an electronic instrument, and the like.
A reduction in power consumption is desired for portable electronic instruments. A liquid crystal display device is generally used as a display device incorporated in such electronic instruments. A plurality of power supply voltages such as a high voltage and a negative voltage are required to drive a liquid crystal display device. In this case, it is desirable from the viewpoint of cost that a liquid crystal driver device which drives a liquid crystal display device include a power supply circuit which generates a plurality of power supply voltages.
Such a power supply circuit includes a voltage booster circuit. As the voltage booster circuit, a charge-pump circuit which generates a boosted voltage by a charge-pump operation is generally used (see JP-A-9-312095, for example). A charge-pump circuit connects one end of a capacitor which stores a charge to various voltages using switching elements (e.g., metal oxide semiconductor (MOS) transistors), thereby boosting the voltage corresponding to the charge stored in the capacitor. Power consumption can be reduced using such a charge-pump circuit.
It is desirable to reduce the output load of the voltage booster circuit as much as possible from the viewpoint of the power efficiency of the voltage booster circuit. Therefore, the output of the voltage booster circuit is directly connected to a circuit to which the output voltage from the voltage booster circuit is supplied, as disclosed in JP-A-9-312095, for example.
A regulator is used when regulating the output potential of the voltage booster circuit. In this case, it is desirable to operate the regulator at a low operating voltage in order to reduce power consumption. Therefore, a voltage regulated using the regulator is input to the voltage booster circuit instead of regulating the voltage boosted by the voltage booster circuit using the regulator.
When regulating a potential using the regulator, the maximum value of the output voltage from the regulator is lower than a high-potential-side power supply voltage of the regulator by at least a threshold voltage AVthp of a transistor, for example. This also applies to the minimum value of the output voltage from the regulator. Specifically, the minimum value of the output voltage from the regulator is higher than a low-potential-side power supply voltage of the regulator by at least a threshold voltage ΔVthn of a transistor. Specifically, the range of the output voltage from the regulator is narrow as compared with the voltage between the low-potential-side power supply voltage and the high-potential-side power supply voltage of the regulator. When the output voltage from the regulator is boosted by a factor of S (S is a number larger than one), only a voltage which is lower in potential by S×ΔVthn than a voltage obtained by boosting the high-potential-side power supply voltage of the regulator can be obtained. This means that the power supply voltage range of a circuit to which a voltage obtained by boosting the output voltage from the regulator is supplied decreases. As a result, a power supply margin decreases.
When the voltage of the voltage booster circuit which boosts the voltage regulated by the regulator exceeds the target voltage, a charge is discharged to a system ground power supply in order to decrease the voltage which exceeds the target voltage. For example, when the absolute maximum rating of an integrated circuit is 6 V and a voltage of 3.3 V is boosted by a factor of two, a charge is discharged so that the potential of the boosted voltage decreases by 0.6 V (=3.3×2-6). Therefore, the power consumption of the power supply circuit can be reduced by reducing the amount of charging/discharging for regulating the voltage which exceeds the target voltage.
The power efficiency of the power supply circuit changes corresponding to the output load of the power supply circuit. This means that a boost operation is performed using unnecessary power depending on the output load. Therefore, it is desirable that the boost capability be changed corresponding to the output load of the power supply circuit, and that the boost capability be changed so that a stable voltage can be supplied to the output load.
Moreover, the output load of the power supply circuit which supplies a power supply voltage to a liquid crystal display device may differ depending on a display image or a display mode of the liquid crystal display device. The boost efficiency of the power supply circuit may decrease when the output load is low.